This application relates to a non-linear PID control algorithm that avoids a potential adverse condition in a vapor compression system.
Refrigerant cycles provide temperature change in a fluid to be treated. In general, a refrigerant cycle includes a compressor for compressing a refrigerant, a first heat exchanger receiving the compressed refrigerant, an expansion device downstream of the first heat exchanger, and a second heat exchanger downstream of the expansion device. Refrigerant flows from the compressor, through the first heat exchanger, through the expansion device, through the second heat exchanger, and back to the compressor. A fluid is heated or cooled at one of the heat exchangers. This basic system can have many uses such as providing hot water, providing air conditioning or providing a heat pump function, among others.
One type of refrigerant cycle is a transcritical cycle. In a transcritical cycle, operation is above the saturation pressure. Thus, there is a degree of freedom with regard to the achieved pressure.
One particular application recently developed by the assignee of this application is for a hot water heating system, wherein the first heat exchanger receives water to be heated. A water pump delivers the water through the first heat exchanger.
As disclosed in co-pending U.S. patent application Ser. No. 10/793,489, filed on even date herewith and entitled “Pressure Regulation in a Transcritical HVAC System,” a control may predict a desired discharge pressure to most efficiently achieve a hot water temperature. A control to achieve the efficient operation monitors a variable with regard to the hot water, and a variable with regard to the refrigerant discharge pressure. These variables are controlled in a manner disclosed in the U.S. patent application Ser. No. 10/793,542, filed on even date herewith and entitled “Multi-Variable Control of Refrigerant Systems.”
The control determines error correction factors for both water temperature and refrigerant discharge pressure, by looking at an error between a desired and actual water temperature and discharge pressure, and both the derivative and integral of these errors.
The basic system 20 is illustrated in FIG. 1, wherein hot water is delivered from a line 21 to a downstream user 22. An input 24 allows an operator of the downstream use 22 to select a desired hot water temperature. It should be understood that the input might not be the selection of a particular temperature, but could instead be the position of a faucet handle, mixing valve handle, etc. Controls for translating these positions into a desired temperature are as known, and would be within the skill of a worker in this art. A sensor 26 senses actual hot water temperature leaving heat exchanger 28. A water pump 30 delivers water through the heat exchanger 28. Feedback from the sensor 26, the control 24, and to and from the water pump 30 are all delivered to an electronic control 32. A sensor 36 senses a discharge pressure downstream of a compressor 34 in a refrigerant cycle 35 associated with the water heating cycle. An expansion device 38 is positioned downstream of heat exchanger 28, and a second heat exchanger 40 is positioned downstream of expansion device 38. The expansion device 38 is controlled by the control 32, and has a variable opening such that the control 32 can open or close the expansion device 38 to control the pressure of the refrigerant within the cycle 35.
In a refrigerant system 35 operating in transcritical mode, there are two different steady state operational cycles available for a given set of ambient conditions. As one moves further to the right in the graph shown in FIG. 2, the operation becomes less efficient. Shown in FIG. 2 is a transition in time between the efficient (good) cycle and inefficient (bad) cycle when traditional control is implemented. The subject of this invention is alternative control that will avoid the transition between one discrete efficient cycle and the alternative inefficient cycle.